Polyalkylene terephthalates, particularly polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), are thermoplastic polyester resins with good electrical properties that are widely used as insulators to cover electrical wires and other electronics components. One criteria expected for the use of thermoplastic resins in electronics is that they must self-extinguish after catching fire as indicated by a UL 94 test rating of V-0. The V-0 test rating includes the requirement that the material tested may not burn with flaming combustion for more than 10 seconds after application of a test flame.
However, as the polyalkylene terephthalates are fairly combustible and, even with the addition of fire retardants (FR), they may not meet the requirements for a UL 94 V-0 rating particularly for thin-walled applications such as thin-walled electrical connectors.
Through the addition of antimony compounds, as a flame poison, typically antimony trioxide, in combination with an organo-halide flame retardant, preferably an organo-bromide, suitable self-extinguishing PBT blends have been developed.
Antimony trioxide by itself is not a fire retardant, and the organo-halide flame retardants, by themselves, are typically insufficient in PBT formulations to achieve a V-0 rating without jeopardizing the mechanical properties of the PBT formulation. However, when combined, they act synergistically to form a very effective flame retardant system for polyalkylene terephthalates.
As a result of this synergy, with low amounts of antimony, equivalent or better fire retardancy can be achieved with substantially reduced amounts of the organo-halide agent.
Thus, to achieve a V-0 rating, PBT formulations frequently require an organo-bromo FR agent, an antimony FR synergist and, to meet the UL 94 V-0 rating drip requirement, a fluoropolymer antidrip component.
Recently, there has been a desire to eliminate antimony trioxide from PBT formulations due to significant increases in its cost as well as human health concerns in some countries. Antimony trioxide has been found, in some studies, to be a potential hazard for human health due to the possibility of skin irritation, lung toxicity, teratogenicity and lung carcinogenicity after repeated exposure.
Consequently, there is an ongoing need for flame retardant polyalkylene terephthalate compositions that are antimony-free, but that maintain or surpass the performance attributes of currently available flame retardant polyester compositions for thin-walled electronics uses.